Hematopoiesis is a highly regulated multistep process. BRCA1, a DNA repair protein well studied for its role in hereditary breast cancer susceptibility, directly interacts with several critical proteins involved in normal hematopoietic stem cell (HSC) function, yet its role in hematopoiesis remains unstudied. Recently, I developed a conditional mouse model with bone marrow-specific deletion of Brca1 in hematopoietic cells and discovered that mice with complete loss of Brca1 function have low blood counts at baseline and spontaneously develop hematologic malignancies. These data provide direct evidence of the importance of BRCA1 for normal hematopoiesis and susceptibility to hematologic malignancies, but the mechanisms remain unknown. The overall objective of this application is to determine how partial and complete BRCA1 deficiencies affect bone marrow function. The work outlined in this grant proposal will determine the specific points during the multistep process of hematopoiesis at which Brca1 acts and whether the defects observed in homozygous Brca1 deficiency are due to abnormalities of differentiation or proliferation (Aim 1). BRCA1 is also emerging as a master regulator of genomic stability. Thus, genomic instability may contribute to malignant transformation in Brca1 deficient hematopoietic cells. Aim 2 will determine whether Brca1 deficient hematopoietic progenitors demonstrate genomic instability either spontaneously or following cytotoxic exposures. Finally, the only commonly observed BRCA1 deficiency state in humans is heterozygous deficiency. Two small studies have found an increase in hematologic toxicities following chemotherapy exposures in individuals with an inherited BRCA1 mutation. Aim 3 will determine whether BRCA1 haploinsufficiency in humans causes detectable deficits in hematopoiesis through 1) a case control study examining the frequency and severity of myelosuppressive episodes following chemotherapy in women with an inherited BRCA1 mutation versus controls; and 2) through studies of the hematopoietic differentiation and proliferation potential of HSCs derived from induced pluripotent stem cells from individuals with inherited BRCA1 mutations as compared with controls. Successful completion of this proposal will provide important insights into the etiology of BMF and HM and a system in which to test novel therapeutic approaches.